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cfdguy May 29, 2002 10:24

transistion
 
can any one help. at what Re does a 2-D channel flow becomes turbulent? suppose i have a obstruction,ie., if study flow past a square cylinder, at what Re the flow becomes turbulent? can any one help? thanx in advance cfdguy

Adarsh May 30, 2002 05:09

Re: transistion
 
depends on the free stream turbulence and other disturbance effects.. such as sound.Critical reynols no. is influenced by many factors in an experiment.Can't say precisely. Adarsh

cfdguy May 30, 2002 07:23

Re: transistion
 
hi adarsh thank you i had done flow visualisation study of flow over a bluff body (sqr cylinder)at re=4000 in a water channel. the picture seems to show smooth flow lines. can i say that the flow is laminar. how can i decide it. bye cfdguy

FVSuppe May 30, 2002 08:39

Re: transistion
 
Look up the book by Cebeci & Smith - 'Analysis of Turbulent Boundary Layers'. There are some correlations and formulas that may be useful to you. As far as I remember there are some formulas to predict start of transition for bluff bodies.

Jongdae Kim May 30, 2002 17:21

Re: transistion
 
1. What is you turbulence intensity at inflow boundary?

2. Aspect ratio ?

3. End condition ? (Do you use end plates which are parallel ?)

4. How to calculate Reynolds number ?

Re=U*D/nu, where U is inflow velocity, D is cylinder section dimension, nu is kinematic viscosity.

In the flow around square cylinder, unsteadiness starts from around Re=45. Around Re=150, another unstability starts. (These Re. numbers may be incorrect.). Around Re=250, drag and lift coefficients seems to be non-periodic, random. Flow of Re<240, plot of time history of drag and lift coefficients are smooth. Flow of Re>250, flow is in the transient stage. These are the experience based on my computational work (LES around bluff body).

At Re=4000, Flow could still be laminar. But in my understanding, it's very difficult or impossible to maintain the flow as laminar unless you intend to keep the flow laminar.

cfdguy May 31, 2002 05:47

Re: transistion
 
hello kim thank you for your message. here r the answers to ur queries. (1)tur.intensity is 1% (2) blockage ratio=(height of cylinder/height of the channel) is (1/14). (3)aspect ratio=(height of cylinder/depth of water) = (1/8)

(3)the end plates are parallel (4)re is same as you defined.and it is 4000. awaiting your reply thank you cfdguy


cfdguy June 6, 2002 09:17

Re: transistion
 
hi all y no more discussion on this topic. bye


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